1. Field of the Invention
The present invention relates to the field of video level control and, more particularly, to a system which automatically monitors and adjusts the video level for a solid-state video camera using a charge-coupled imaging device (CCD).
2. Description of the Prior Art
One of the problems associated with the use of any television or video camera equipment, especially that using CCD imaging devices, is that of controlling the level of video output signal in response to changes in the light level entering the camera lens system. The video output level of the camera must be kept within an acceptable range in order that that quality of the video images produced may be maintained. Of course, the problem is especially acute in situations where the light level cannot be controlled.
There have been several approaches to controlling the video output level for such devices. In one approach the amount of light transmitted through the TV camera lens is controlled by mechanically varying the iris opening of the lens in response to changes in the environmental light level sensed by the system. These normally consist of servo motors and motor controls which are used to drive the iris toward an open or shut position in response to variation in the sensed input light level. The control may be separate or may be in the form of an override control which overrides the normal iris control system which operates in response to image contrast level. In another system approach, a high speed shutter which may be driven by a pulsed width modulator is utilized to vary the average length of time to which the photo-sensitive devices of the camera are exposed to the incoming light during the composition of the frame. This, of course, also requires a mechanical shuttering device with associated drive circuitry and mechanism.
An additional approach which has been used in an effort to control the camera output is accomplished through an adjustable gain control system within the camera signal processing system. This may be in the form of an automatic gain selecting unit which selects one of several gain ranges in response to the lens-iris setting of the camera device or it may be an intermittent system which averages the total intensity of the final image.
While the above devices have been somewhat successful, all suffer from certain undesirable drawbacks. The mechanical lens and shutter operating systems require complicated electro-mechanical servo systems and are subject to all the limitations of mechanically operated devices. The automatic gain control devices eliminate the problems associated with mechanical iris or shutter operation. However, these systems introduce problems such as image distortion associated with wide-ranging variation in the signal gain characteristics.